Supporting MSVC cl in ax_cxx_compile_stdcxx

[Paul Smith]

I see that the ax_cxx_compile_stdcxx macros support checking for
compiler arguments in the following formats:

  * -std=c++XX
  * +std=c++XX
  * -h -std=c++XX

Has any thought been given to supporting the format used by cl (the
Visual Studio compiler)?  That would be either (they are equivalent):

  * -std:c++XX
  * /std:c++XX

?  Annoyingly it requires "std:" rather than "std=".